Fundus photographic technique to determine eye refraction for optic disc size calculations

ABSTRACT

The determination of magnification in an ophthalmic fundus camera image is achieved by detecting the focusing mechanism position, and using a calibration function to calculate the magnification from the focus position.

The present application claims the benefit of priority under35USC§119(e) of U.S. patent application Ser. No. 60/181,334 filed Feb.9, 2000.

FIELD OF THE INVENTION

The present invention relates generally to ophthamological retinalimaging, and in particular to a method and apparatus for determining anobject size, such as the optic disc or other retinal object, in an imagefrom a fundus camera.

BACKGROUND OF THE INVENTION

In ophthalmic imaging, the determination of scale is a challenge for theophthalmologist using a fundus camera.

For example, the determination of the true size of the optic nerve headfrom its image on a fundus photograph is of great clinical importance inthe diagnosis of glaucoma. As is well recognized in the art, glaucoma isa visually debilitating disease process resulting in damage to the nervetissue of the optic nerve and having prevalence in the over 40 age groupof about 1% (see Duane's Clinical Ophthalmology, Tasman W and Jaeger Eeditors, Vol. 3, Chapter 42, Glaucoma: general concepts).

The current methods for determining optic disc size include performingan ultrasound of the eye to determine the axial length or diameter ofthe eye and from this value calculating the image magnification. Anothermethod uses the optical error of the eye to calculate imagemagnification. A further method includes measures of the curvature ofthe cornea in addition to optical error of the eye (front surface of theeye) to better refine the accuracy of the technique using the opticalerror accuracy in calculating the image magnification (see Bengtsson Band Krakau C. E. T. “Correction of optic disc measurements on fundusphotographs”, Graefe's Arch Clin Exp Ophthalmol, 1992; 230: 24-8). Thesetechniques are tedious as they require photograph and supplementaltesting either to determine the axial length of the eye or its opticalerror (glass refraction) or its optical error and the curvature of thecornea.

The diagnosis of glaucoma from a fundus photographic image is aided byknowing the true size of the optic nerve head and the true area of thenerve tissue visualized or detected on it (Jonas et al, InvestigativeOphthalmology and Visual Science, 2000). As reported by Jonas et al.,the vertical cup-to-disc ratio corrected for optic disc size, the totalneuroretinal rim area, the rim-to-disc area ratio, and the cup-to-discarea ratio corrected for disc size are the most valuable optic discvariables for early detection of glaucomatous optic nerve damage.Correction for optic disc size is necessary for optic disc variablesdirectly or indirectly derived from the optic cup. However, as alludedto above, not all eyes are the same in terms of their optical errors andsizes which results in differences in image magnification on the fundusphoto. This means that to calculate true sizes of objects on the fundusphoto, the methods described above must be employed. This can beimpractical.

Papastathopoulos and Jonas have also reported on efforts to evaluate theoptic disc size using a slit lamp (see “Ophthalmoscopic assessment ofthe size of the optic nerve papilla”, Ophthalmoskopische Abschatzung derGrosse der Papilla N. optici., Klin Monatsbl Augenheilkd 1997Nov;211(5):291-5). While feasible, such measurement methods arecumbersome and not easily put into everyday practice.

There are other retinal structures in the fundus photograph which canalso be measured in absolute units, and these structures include retinalblood vessels (both normal and abnormal), tumors, hemorrhages, andexudates, to name a few. Although absolute sizing of these structuresfrom the fundus photograph is not, as of now, as diagnosticallyimportant as the sizing of the optic nerve head, the ability to performthis function could well lead to new disease diagnostic criteria.

SUMMARY OF THE INVENTION

It is a goal of the present invention to provide a simple means toperform the mentioned magnification calculation to permit thedetermination of true sizes of optic nerve and optic nerve structures inorder to allow for rapid screening for glaucoma from a simple fundusphotograph. Early diagnosis of glaucoma would decrease patientdisability and hence have important socioeconomic benefits.

It is a goal of the present invention to provide an expedient method fora good approximation of the true sizes or dimensions of objects ofinterest as viewed on a fundus image.

It is a goal of the present invention to provide a method and apparatusfor determining, in an easy and reliable manner, a scale of an image andthus an object size, such as the optic disc or other retinal object, inthe image from a fundus camera.

It is a further goal of the present invention to provide a method andapparatus for controlling for inter-session eye-camera magnificationvariability by helping place a fundus camera in front of the eye.

According to the invention there is provided a method ofophthalmological imaging comprising the steps of positioning a patient'seye before a fundus imaging apparatus, adjusting a focus mechanism ofthe imaging apparatus to bring a retina of the eye into focus, recordinga position setting parameter value of the focus mechanism as adjusted inthe previous step, acquiring an image using the fundus imagingapparatus, determining a magnification of the image using the positionsetting parameter value and a calibration function established for thefocus mechanism, and displaying the image with an indication of themagnification.

Preferably, the step of displaying comprises superposing a scale indexmarker on the image, and the step of adjusting comprises manuallyoperating a focus mechanism to bring the retina into focus. Preferably,the step of displaying comprises displaying the image on a screen, andthe step of displaying comprises printing the image.

The steps of recording, determining and displaying may be carried outautomatically by electronic means.

The method may also further comprising steps of re-positioning thepatient's eye before a fundus imaging apparatus at a time of laterexamination, setting a focus mechanism of the imaging apparatus to thesetting parameter value, adjusting a position of the imaging apparatusto bring the retina of the eye into focus, acquiring a second imageusing the fundus imaging apparatus, and displaying the second image withan indication of the scale.

According to a further aspect of the invention, there is provided amethod of obtaining at least one parameter useful in diagnosing glaucomacomprising the steps of positioning a patient's eye before a fundusimaging apparatus, adjusting a focus mechanism of the imaging apparatusto bring a retina of the eye into focus, recording a setting parametervalue of the focus mechanism as adjusted in the previous step, acquiringan image using the fundus imaging apparatus, determining a scale of theimage using the setting parameter value and a calibration functionestablished for the focus mechanism, and measuring true dimensions of anoptic nerve head and nerve tissue of the eye using the image and thescale, calculating at least one parameter indicative of glaucoma usingthe true dimensions.

The step of adjusting may comprise manually operating a focus mechanismto bring the retina into focus. The steps of recording, determining,measuring and calculating may be carried out automatically by electronicmeans.

The method preferably further comprises steps of re-positioning thepatient's eye before a fundus imaging apparatus at a time of laterexamination, setting a focus mechanism of the imaging apparatus to thesetting parameter value, adjusting a position of the imaging apparatusto bring the retina of the eye into focus, acquiring a second imageusing the fundus imaging apparatus, measuring true dimensions of anoptic nerve head and nerve tissue of the eye using the second image andthe scale, and recalculating at least one parameter indicative ofglaucoma using the true dimensions from the second image.

According to another aspect of the invention, there is provided a funduscamera comprising a focus mechanism, and measuring means associated withthe focus mechanism for measuring a position setting parameter of thefocus mechanism.

The measuring means may comprise a vernier scale on the camera focusingcontrol knob. The measuring means may also be electronic and provide aposition setting parameter signal, and the camera may further comprisemeans for automatically determining a magnification of an image usingthe signal and a calibration function for the focus mechanism.

The camera may also further comprise an image storage means for storingthe image, and an image displaying means for displaying the image withan indication of the magnification.

BRIEF DESCRIPTION OF THE DRAWING

In the sole drawing, there is illustrated schematically a fundus camerahaving a focus mechanism with a scale acquiring a retinal image of aneye, according to a preferred embodiment of the invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

According to the invention, the position of the focusing knob ormechanism on the fundus camera is recorded, which position reflects theoptical error or glass refraction of the eye. The glass refraction ofthe eye is used to calculate the object to image size ratio ormagnification. This focus mechanism position is preferably automaticallyincorporated into a means to calculate the eye-camera magnification toarrive at a good estimate of the true or absolute measurements ofretinal structures on the fundus photograph, such as the optic nerve,its components including the neuroretinal rim area and the cup. All ofthese measurements are important in the diagnosis of glaucoma.

The Applicant of the present application has noted that the observationand photography systems of the fundus camera rely on the principle ofindirect ophthalmoscopy (“Some Essential Optical features of the ZeissFundus Camera”, Bengtsson B and Krakau C. E. T. Acta Ophthalmologica Vol55, 1977:123-131). The location of the intermediate real image of thefundus as created by the front lens of the camera depends on the opticalpower of the eye being photographed. The setting or position of thefocusing mechanism on the camera to best see the intermediate image alsoreflects the glass refractive error of the eye. This measure of theglass refraction of the eye can then be used to calculate the eye-cameramagnification factor produced on a fundus image. If, for example, thesize of the optic disc is desired, this eye-camera magnification factorcan be used along with the area of the disc occupied in the image (e.g.the area can be measured in pixels using image processing software, suchas Adobe PhotoShop™) to arrive at an area measurement of the disc. Thisarea measurement can then be corrected to yield a good approximation ofthe true disc area with “correction factors” (see Rudnicka et al.“Magnification Characteristics of Fundus Imaging Systems” OphthalmologyVol 105, Number 12, December 1998; 2186-2192) or deriving “correctionfactors” from a standard group of subjects whose disc area has beendetermined by techniques as described above.

In tests performed, it has been found that the relation between focusmechanism position and glass refraction is highly correlated. Twotelecentric fundus cameras—the Topcon TRC-50F50FT and the Topcon TRC-50xwere used to perform twenty degree red free photographs of the opticnerve in twenty subjects (N=11 with the Topcon TRC-50F50FT and N=9 withthe Topcon TRC-50x). A vernier scale was attached to the focussing knobwhich permitted a measurement of the knob position to be recorded. Acorrelation of this measurement with the eye refractive error wasperformed. The position of the focusing knob on both cameras correlatedhighly with the refractive error of the eye being photographed r=0.97for the TRC-50F50FT and r=0.99 with the TRC50X. It should be noted thatthe photographs must be done in such a fashion so as to minimize theeffect that the photographer's own lens accommodation may have on thefocussing of the image. This can be done by having a photographer ofsufficient age so his own diminished accommodative powers will notinterfere with the focussing of the camera, or by ensuring thatfocussing the camera in done in such a way as to not employ one's ownfocussing ability. This problem will be not be important in cameras thatemploy an automatic or semi-automatic electronic focussing mechanism.

The position of the focussing mechanism reflects the optical refractiveerror of the eye being photographed which in turn can be used tocalculate the eye-camera magnification. This position measurement can beincorporated into a simple method to calculate optic disc or otherretinal object size. Although the above example was performed withtelecentric cameras, some changes in this simple technique for retinalobject size determination will also permit magnification factors fornon-telecentric cameras to be calculated (Rudnicka et al. MagnificationCharacteristics of Fundus Imaging Systems. Ophthalmology Vol 105, Number12, December 1998; 2186-2192).

Currently, the measurement of fundus (retinal) objects from aphotographic image captured with a fundus camera is done by calculatingmanually the eye-camera magnification system with the formulas whichemploy either the length of the eye, the glasses strength of the eye, orthe glasses strength and corneal curvature as mentioned above. Alsoknown from the literature is that the known value of the glassesstrength of the eye can be used to set the position of the focusing knobto bring the image of the retina into clear focus when taking thephotograph of the retina, without adverse affect resulting from thephotographer's own accommodation (see the mentioned Bengtsson and Krakau(1997), p. 131).

According to the present invention, the position of the focussingmechanism on the retinal camera is used directly to calculate theeye-camera magnification. This magnification factor can in turn also beused along with a simple software measuring tool whose scale changesaccording to the eye-camera magnification to calculate retinal objectsize (e.g. the optic nerve and the optic cup which are indices fordiagnosing glaucoma, vessel caliber or tumor diameter). This is usefulfor rapid screening of large numbers of people especially withautomation of photograph reading.

As will be appreciated, the method according to the invention maycomprise the following steps as described herein below with reference tothe appended drawing illustrating schematically a fundus optical imagingsystem.

The patient's eye is positioned for retinal photograph (or optic nerve).The aerial image of the object is focused on the film or CCD (imageplane) with the focusing mechanism. The position of the focusingmechanism is recorded with the image. This can also be done by automaticmeans, such as a position sensor measuring the position of the focusingmechanism and a digital signal can be obtained for providing a focusingposition measurement. The recorded position is then calibrated to theeye optical error (glass refraction) and is used to calculate theeye-camera magnification. An accurate scale of the fundus image can bedetermined using the eye-camera magnification calculated in the previousstep.

When measuring objects found in the image using a software imageanalysis tool, the scale determined in the previous step is used tomeasure linear objects (e.g. vessel widths) or two-dimensional objects(e.g. the optic nerve head) in absolute units (e.g. In mm or mm²). Theposition of the focusing mechanism recorded previously can be used infuture photographs of the same patient to help control for inter-sessioneye-camera magnification variability by helping place the camera infront of the eye.

However, once the diagnosis of an ocular condition necessitating afundus photograph is made (e.g. glaucoma) in a given patient, theeye-camera magnification is constant (unless of course the patient hashad surgery to correct for myopia or cataract surgery or develops acondition such as a cataract which could change the glass refraction ofthe eye). As mentioned, the position of the focusing mechanism on thecamera can be used to help minimize variability of magnification betweenphoto sessions. For each session, the patient sits in front of thecamera and the camera is manually moved to a fixed distance (say 10 cm)from the patient which is determined by the photographer. There is acertain error in this positioning owing to its manual input. This errorcan cause a change in the position of the focusing mechanism and hence achange in eye-camera magnification. Knowing the position of the focusingmechanism of the previous photographic session will permit a decrease ininter-session image magnification variability.

1. A method of ophthalmological imaging comprising the steps of:positioning a patient's eye before a fundus imaging apparatus; adjustinga focus mechanism of said imaging apparatus to bring a retina of saideye into focus; recording a position setting parameter value of saidfocus mechanism as adjusted in the previous step; acquiring an imageusing said fundus imaging apparatus; determining the glass refraction ofsaid patient's eye using said position setting parameter value and acalibration function established for said focus mechanism; calculating amagnification from said glass refraction of said patient's eye; anddisplaying said image with an indication of said magnification.
 2. Themethod as claimed in claim 1, wherein said step of displaying comprisessuperposing a scale index marker on said image.
 3. The method as claimedin claim 1, wherein said step of adjusting comprises manually operatinga focus mechanism to bring said retina into focus.
 4. The method asclaimed in claim 1, wherein said step of displaying comprises displayingsaid image on a screen.
 5. The method as claimed in claim 1, whereinsaid step of displaying comprises printing said image.
 6. The method asclaimed in claim 1, wherein said steps of recording, determining anddisplaying are carried out automatically by electronic means.
 7. Themethod as claimed in claim 1, further comprising steps of:re-positioning said patient's eye before a fundus imaging apparatus at atime of later examination; setting a focus mechanism of said imagingapparatus to said setting parameter value; adjusting a position of saidimaging apparatus to bring said retina of said eye into focus; acquiringa second image using said fundus imaging apparatus; and displaying saidsecond image with an indication of said magnification.
 8. A method ofdetermining the true dimensions of an optic nerve head and nerve tissueof a patient's eye, the method comprising the steps of: positioning apatient's eye before a fundus imaging apparatus; adjusting a focusmechanism of said imaging apparatus to bring a retina of said eye intofocus; recording a setting parameter value of said focus mechanism asadjusted in the previous step; acquiring an image using said fundusimaging apparatus; determining a glass refraction of said patient's eyeusing said setting parameter value and a calibration functionestablished for said focus mechanism; determining a scale of said imageusing said glass refraction of said patient's eye; and measuring truedimensions of an optic nerve head and nerve tissue of said eye usingsaid image and said scale.
 9. The method as claimed in claim 8, whereinsaid step of adjusting comprises manually operating a focus mechanism tobring said retina into focus.
 10. The method as claimed in claim 8,wherein said steps of recording, determining, measuring and calculatingare carried out automatically by electronic means.
 11. The method asclaimed in claim 8, further comprising steps of: re-positioning saidpatient's eye before a fundus imaging apparatus at a time of laterexamination; setting a focus mechanism of said imaging apparatus to saidsetting parameter value; adjusting a position of said imaging apparatusto bring said retina of said eye into focus; acquiring a second imageusing said fundus imaging apparatus; measuring true dimensions of anoptic nerve head and nerve tissue of said eye using said second imageand said scale; and.
 12. A fundus camera comprising: a focus mechanismfor focusing a fundus camera to image a retina; measuring meansassociated with said focus mechanism for measuring a position settingparameter of said focus mechanism and providing a position settingparameter signal; and means for automatically determining the glassrefraction of patient's eye of said image using said signal and acalibration function for said focus mechanism.
 13. The camera as claimedin claimed in claim 12, wherein a magnification of said image is afunction of said glass refraction of said patient's eye, furthercomprising: an image storage means for storing said image; and an imagedisplaying means for displaying said image with an indication of saidmagnification.
 14. A method of calculating the glass refraction of apatient's eye, the method comprising the steps of: positioning apatient's eye before a fundus imaging apparatus; recording theseparation distance between said patient's eye and said imagingapparatus; adjusting a focus mechanism of said imaging apparatus tobring an image of a retina of said eye into focus; recording a positionsetting parameter value of said focus mechanism as adjusted in theprevious step; and calculating the glass refraction of said patient'seye using said separation distance and said position setting parametervalue; displaying said image.